A polymer PTC thermistor is a device which allows the flow of current, or which reduces current flow by using a positive resistance temperature characteristic, that is, a PTC (Positive Temperature Coefficient) of a conductive polymer which varies in conductivity due to thermal expansion. In detail, it is common that in a conductive member which is composed of a conductive polymer, ceramic or the like, the conductive member is soldered to two electrodes disposed on two separate positions thereof.
The conductive polymer is one type of material which composes the conductive member. The conductive polymer is a polymer resin prepared, for example, by kneading polyethylene and carbon black together and crosslinking thereafter with radiation. Within the conductive polymer, the carbon black particles are linked to each other in a room-temperature environment so that numerous conductive paths are formed through which current flows, and good conductivity is exhibited. However, when the conductive polymer expands thermally due to excessive current flowing in the conductive paths or an increase in the ambient temperature, the distances between the carbon black particles are increased, thus severing the conductive paths, and the resistance rises sharply. This is called the positive temperature coefficient of a conductive polymer, or PTC. This invention utilizes this characteristic.
The polymer PTC thermistor can be used as a switch using as a trigger the magnitude of current flowing between the electrodes disposed on two separate positions of the conductive member. When an overcurrent occurs between the electrodes, the conductive member thermally expands due to self-heating caused by Joule heating, and the distances between the carbon black particles are increased, thus the conductive paths are severed. As a result, it becomes difficult for current to flow. When current flow between the electrodes is cut off, the conductive member shrinks due to stoppage of the self-heating, and the distances between the carbon black particles are decreased, thus conductive paths are formed. As a result, the PTC thermistor returns to a state enabling the passage of current.
Also by utilizing the PTC characteristic of the conductive member, a PTC thermistor as described above may allow current of a prescribed size or less (called the hold current) to flow if the ambient temperature is lower than a prescribed temperature (the temperature at which the conductive member will thermally expand). If the ambient temperature reaches the prescribed temperature or higher, the conductive member will thermally expand so that it becomes difficult for current to flow. In this way, it may also function as a switch using the change in the ambient temperature in which the conductive member is placed.
Japanese Unexamined Patent Application, First Publication No. H06-163203, describes a conductive paste whose conductivity changes in response to temperature change.
When a PTC thermistor as described above is installed in a circuit, there is no occurrence of a problem when it is used in a proper state. However, it has been pointed out that if the device is activated for a long time owing to an overcurrent, or if it is placed in a very high temperature environment for a long time, the conductive member may fail and the electrode may be short-circuited.
This invention was made in view of the above circumstances and is intended to prevent a short between the two electrodes to secure the safety of the circuit.